1. Field of Invention
The present invention relates generally to injection molding apparatus. More particularly, the invention relates to an improved cone tip bushing for use in an injection molding apparatus.
2. Summary of the Prior Art
Various forms of so-called "runnerless", injection molding apparatus are widely used and well known in the art. Such apparatus allows the molder to produce molded articles with only small "vestiges" (i.e., small protrusions of solidified melt material) attached thereto. Accordingly, large sprues or runners attached to completed articles formed by the apparatus are avoided. This provides savings in both trimming labor and material waste. Further, apparatus cycle times are reduced, thereby increasing the efficiency of the injection molding operation. Despite these advantages, however, problems remain.
In "runnerless" injection molding apparatus, molten melt material is intermittently injected into a manifold under pressure for transmission through internal manifold passageways (runners) and bushings. The bushings extend into outer cavities in the surface of the mold to locations substantially adjacent to gates (i.e., passageways having small transverse cross-sections). The gates connect the bases of the outer cavities to inner article formation cavities of the mold. Pressurized "shots" of melt material are ejected from the distal ends of the bushings into the space between the bushings and the adjacent gates (i.e., into the "gate areas"), and thereafter are injected into the inner article formation cavities of the mold through the gates.
The basic concept of such "runnerless" injection molding apparatus is to maintain the melt material in the manifold and the bushings in a molten state throughout the operation of the apparatus, while at the same time allowing melt material injected into the article formation cavities of the mold to solidify and be ejected from the mold as completed articles. To accomplish this, both the manifold and the bushings are maintained at the desired melt temperature, while the mold is cooled. Accordingly, in order for the apparatus to function efficiently, means must be provided which substantially isolate the heated and cooled elements of the apparatus from one another.
The isolation means typically includes support pads located between the bottom manifold surface and the upper mold surface. These support pads provide a spaced, structural contact between the hot manifold and the cold mold. They also are made of material having a low thermal conductivity. Therefore, an air gap is created between the lower surface of the manifold and the upper surface of the mold. That air gap, and those support pads, together act to thermally isolate the hot manifold from the cold mold.
The isolation means also comprises air gaps and/or other insulating means located between the side walls of the outer cavities in the mold and the outer surfaces of the bushings. More particularly, the bushings commonly extend downwardly from the bottom surface of the manifold, through the air gap created by the support pads, and thence into the outer cavities in the mold. The bushings are so sized and mounted as to be spaced from the sidewalls and the bases of the respective outer mold cavities.
In addition, features such as (i) slots in the manifold radiating from the proximal ends of the bushings mounted in the manifold, and (ii) centering protrusions extending axially upwardly from the bases of the outer mold cavities which are adapted to engage the respective bushings proximally of the adjacent gate areas, are often incorporated into the apparatus design. The slots allow the bushings to remain centered relative to their respective associated outer mold cavities despite thermal expansion and/or contraction of the various other elements of the apparatus. The centering protrusions, on the other hand, assure the alignment of the bushing lumens with the respective gates.
Typically, the centering protrusions are formed of titanium, or some other low thermal conductivity material. Further, the contact area between the protrusions and the bushings is kept small. Still further, those contact areas are substantially spaced from the gate areas. These features minimize the detrimental effects of direct thermal contact between the bushings and the mold.
In some cases, bushings have been developed which include a bore defining portion having a small gate at its bottom end. In such cases, the mold maker is relieved of the responsibility of creating components which must be machined to very close tolerances for receipt of the bushings. Instead, the mold is simply provided with appropriately sized outer cavities which may either open directly into the inner article formation cavity of the mold, or may be connected to the article formation cavity by a separate gate. In both cases, the bore defining portions of the bushings are press-fit into, or otherwise secured within, the appropriately sized outer cavities so as to effectively become part of the mold.
It further is well known in the art that the relative geometric configurations of, and the composition of, the bushing tip and the mold in the vicinity of the gate must be carefully selected and controlled in conjunction with the material being molded. This is particularly the case in open bore type injection molding apparatus.
Specifically, it is these features which allow the mold to pull heat from the gate area during reduced melt pressure portions of the injection molding cycle. Therefore, it is the appropriate balance among these features which allows an approximation of the optimum, axially extending thermal gradient (profile) within the melt located in the gate area to be provided. This thermal profile, during the low pressure portion of the injection molding cycle, determines the size and shape of the so-called "vestige" which is left projecting from the completed article upon its ejection from the mold.
Without such control, undesirable stringing of the melt material between the gate and the article at the break point between the molten material and the so-called "vestige"; drooling of melt material from the gate outlet subsequent to formed article ejection from the mold; and/or freeze off of the gate area may take place. These events may result in ruined parts, material waste, the need for unnecessary trimming operations and/or undesirable machine down time to free the various channels of frozen (i.e., solidified) melt material.
It has been found that computer modeling by a method known as finite element analysis may be used to advantage in making the above-referred-to selections. This is particularly important because the relationships between the various parameters which determine thermal gate control are not linearly related to one another. Accordingly, the ultimate effect upon overall apparatus operation occasioned by varying the material of a particular component defining the gate area, or the dimensions thereof, cannot be readily predicted.
Further, in some applications it is deemed critical not only to avoid stringing and/or drooling from the gate area, and gate freeze off, but also to minimize the size of the "vestige". In conventional valve gated apparatus, this is accomplished by the distal end of the axialy reciprocating valve pin filling the gate volume after each "shot" of melt material. In open bore apparatus, on the other hand, it has been found to be advantageous to provide the bushings with a heated, generally cone shaped tip. The pointed end of this tip is adapted to project into the gate so as to provide additional heat input to the center of the gate channel. Heretofore, this has been achieved in various ways with varying degrees of success.
In one such alternative, elongate, heated probe elements (sometimes referred to as "torpedos") having pointed distal ends have been axially located in the melt flow lumens of the bushings. In such apparatus, the pointed ends of the probes extend beyond the distal ends of the bushings so as to reside in the gates. Such probe elements, however, cause undesirable non-uniformities in melt material flow. Specifically, the melt is forced to flow through the bushing lumens in the annular channel surrounding the probe. This creates serious and undesirable pressure losses along the length of the bushing lumen.
Further, other problems arise because of the fact that such probes are generally heated by electrical resistance heaters. Electrical resistance heaters inherently display a temperature gradient along their lengths. Accordingly, their use creates regions which are hotter, and regions which are colder, than the optimumally desired melt temperature along the length of the bushing lumen. Therefore, it is necessary to adjust the temperature output from the colder regions so as to assure that melt freeze off in those regions does not occur. As a result, however, the temperature of the hotter regions is also increased. This may cause unacceptable melt material degradation or burning in the hotter regions.
Another alternative is to taper the distal end of the bushing so as to form a distally pointed cone shaped section. This may be accomplished by mounting an insert containing an axially disposed, solid, cylindrical element having a pointed end in the distal end of the bushing lumen such that the pointed end of the cylindrical element extends through the gate area and into the gate. It also may be accomplished by forming the distal end of the bushing itself in the shape of a distally pointed cone which is sized for extension through the gate area and into the gate of the assembled apparatus.
The first of these alternatives forces the melt to flow through an annular passageway in the insert, and thereby creates non-uniformities in the melt flow and undesirable pressure losses. Further, in one form of such an apparatus, a helical baffle has been located between the cylindrical element and the side wall of the insert's lumen in an attempt to minimize the non-uniformities of melt flow through the insert. The baffle, however, imparts a non-axial component to the distally directed melt flow. Conventional injection molding operations are based upon a substantially totally axially directed and homogeneous melt flow through the gate.
Further, the helical baffle tends to create "dead spots" along the melt flow path. Melt material may accumulate in these "dead spots". Accordingly, the presence of the baffle can lead to the introduction of degraded, burned, solidified or off color melt material into the main melt flow through the gate area and the gate. Such material may clog the gate. It also may ruin completed molded articles by permiting substandard material or streaks of off color melt to be incorporated into such articles. The streaking problem is particularly troublesome following a change in the color of the source melt material.
In the second of the latter alternatives, the distally pointed section of the cone shaped section of the bushing resides in the gate area and gate of the assembled apparatus. A plurality of substantially equally, circumferentially spaced portals extend through the side wall of the cone shaped section adjacent to its larger end so as to connect the bushing lumen with the gate area. The melt material flowing through the bushing lumen divides adjacent to the distally pointed section and exits the bushing through the portals.
Thereafter, the melt material ejected from each portal flows primarily directly and distally toward the gate. The melt material, however, also flows secondarily in a spreading manner circumferentially along the outer surface of the cone shaped bushing tip so as to ultimately fill at least the gate area of the apparatus. When this occurs, melt material may accumulate in dead spots located in the spaces between the portals (i.e., in regions adjacent to the larger end of the conical tip between the primarily distally flowing melt streams from the respective portals). This in turn may lead to degraded, burned, substandard or off color melt entering into the main melt stream so as to cause unacceptable streaking or other defects in completed articles over a substantial period of time.
It also has been found that the melt flow in the second of the latter alternatives results in the presence of unacceptable so-called "striation lines" in completed molded articles. These "straition" lines radiate in spaced relation to one another outwardly from the vestige protruding from the outer surface of the article. The spacing of these "striation" lines corresponds to the locations of planes which are perpendicular to the outer surface of the cone shaped distal tip and which extend radially outwardly from the longitudinal axis of the bushing substantially midway between each pair of portals.
The reasons for the formation of these "striation lines" are not fully understood. The cone shaped bushing tip is heated, but it also resides in the thermally controlled gate area wherein the transition of the melt from a molten, substantially liquid form to a solidified form occurs. It is theorized, therefore, that due to the primarily distal direction of the flow of melt material from each portal in the thermally controlled transition zone, the molecular chains of the polymeric melt material tend to become generally axially aligned with one another as the material flows distally along, and spreads circumferentially around, the cone shaped tip. Therefore, when the circumferentially spreading portions of the melt flow come into contact with one another, they tend to weld (or "knit") together in a manner in which the aligned molecules cause the formation of a weak joint. This is to be distinguished from the more desirable homogeneous merging of the molecules with one another in a manner in which the molecular chains of the respective speading portions of the melt flow intertwine with one another. This welding is believed to be the source of the undesirable "striation lines".
Still further, until recently, injection molding apparatus wherein both (i) the volume of the air gap between the outer bushing surface and the adjacent side wall of its associated outer mold cavity is maximized, and (ii) the volume adjacent to the gate area into which molten melt material is allowed to flow during pressurized portions of the injection molding cycle is minimized, were not available in a single apparatus. This is because it was not believed to be possible to locate a seal in the gap between the bushing and the mold in close proximity to the gate area. Specifically, it was conventionally believed that such a seal would create a heat passageway between the bushing and the mold which would detrimentally affect the thermal control of the gate area and gate. Accordingly, since in conventional injection molding apparatus at least a substantial portion of the gap between the outer surface of the bushing and the side wall of the outer mold cavity contained a material less insulative than air, completed part quality was compromised.
In addition, due to the compression/decompression pumping forces applied to melt material located in the gap between the bushing and the mold by the cyclical application and release of pressure on the melt material flowing through the apparatus, the main melt flow was subject to contamination by degraded melt material or pieces of solidified melt material. This problem was particularly significant in those instances wherein the color of the source melt material was changed. The reason for this was that until the melt material of the first color located in the space between the bushing and the mold was cleared by the above-referred-to pumping action, completed articles formed by the apparatus contained streaks of the first melt color. Such streaked articles obviously had to be disposed of as waste. Also, in severe cases, the apparatus had to be disassembled to clear the apparatus of off color melt material.
The latter problem has recently been solved to the extent possible by the invention of our co-pending U.S. patent application Ser. No. 08/105,799, filed Aug. 12, 1993. That invention provides a thin-walled metallic seal in close proximity to, and surrounding, the gate area of the apparatus without introducing adverse affects upon the thermal control of the gate area. Hence, the number of operational cycles required to clear an apparatus including a conventional cone tip bushing and the above-referenced seal of off color melt (e.g., for the apparatus to stop producing streaked output articles) was reduced by an order of magnitude. For example, in one case completed article streaking ceased after about 150 apparatus cycles of a conventional apparatus alone, while in an equivalent apparatus containing a seal in accordance with the invention of our above-referenced patent application streaking ceased after about 50 apparatus cycles.